Conventionally dimmers work by chopping up each period of a sine wave of an AC supply into a portion in which the load is connected to the supply (on) and a portion in which it is disconnected (off). One approach is to turn the supply to the load off whenever the sinusoid of the supply passes through zero, and then turn it back on again at a voltage determined by the setting of a control knob or the like. Another approach is to turn the supply to the load on whenever the supply passes through zero and then turn it off at a voltage determined by the setting of a control knob or the like. The proportion of the time during the AC cycle (the “duty cycle”) and hence the power delivered to the load is therefore controlled by the control knob. In some prior art dimmers, a triac is used as a switch to turn the supply to the load on and off, with the gate thereof being connected to a voltage controlled by the control knob. Others use a MOSFET which have a small voltage drop between source and drain and so provide low power loss.
A problem with dimmers is related to low voltage lighting that uses an “electronic transformer”. Low voltage lamps are, of course, not directly connected to the mains supply, but rather are typically powered by a transformer, usually an “electronic transformer”. As is known, electronic transformers typically derive a low voltage AC supply via an oscillator that is powered by the mains supply, often with the oscillation of around 40 kHz controlled by a diac (Diode for Alternating Current) or the like. The problem with these transformers is that they often generate jitter, which is fed back to the AC supply. In particular, if the supply to the electronic transformer is via a dimmer, the zero-crossing of the supply across the dimmer often jitters around the true zero crossing of the voltage of the mains supply, because the switching of the electronic transformer is not synchronous to the mains supply. This can be a problem, if as in the present invention, the dimmer bases its switching timing on measurements of the timing of the zero crossing, which are affected by this jitter. In turn, this affects the power delivered, causing the lamps to flicker, which is often unacceptable to users. The flicker is often caused by the window for which the lamp is on being offset differently with respect to the sine wave of the main supply in each cycle thereof.